I have three experiences, in three very different settings, across a span of 15 years, with having two providers at the same time. In ALL three instances, it was a disaster as the two wound up paying near full attention to bantering with each other in their language which I didn't understand, more or less completely ignoring me. So the value for two was about 10% the value of just one. How common is this? Any advice?

...That way, when they ignore you and talk to each other, at least you'll be able to understand what they are saying.

Getting women that already knows each other
and have played together.  

Try a VIP duo who already love each other!

and let her recommend the other provider. Also helps if the duo has reviews.  Seeing two unknown providers to you, can be a recipe for problems, such as you've experienced.

I have a Sexy Busty Doubles Partner  
we know each other very well  
been friends for 2years  
we like to take our time and find out what makes you tick ..
 
just check us out sometime  
 Gina  

I just had my first doubles with 2 providers that regularly work together.  IMO save up and get two ladies from TER that know (and play with) each other well.  PM me if you want the info.  IMO stay away from AMP doubles, its not the same at all

Could not agree more. It's always best to let a provider choose the other girl. Things run much smoother and there is less competition between girlfriends.